Our documentations shows how to add 3rd party plugins for Android platform and this is possible as shown in our documentation.

But I think it's a good idea to add a note about that this won't work for iOS platform at the moment.

And just to clarify this: you can build your app in Appery.io and when time will come for testing on the device you can export your project to xCode and add any plugin to your app or even write your own.